1. Field of the Invention
This invention relates to a method for recovering high viscosity petroleum from an underground reservoir. More specifically, the invention relates to oil recovery methods wherein heated aqueous fluids are injected into directionally drilled injection wells which are drilled radially inward towards a central producing well, the bottom hole location of said injection well being substantially less than the normal spacing between a conventional pattern of producing wells and the surface location being not less than the normal spacing interval between production wells.
2. Description of the Prior Art
A great many subterranean petroleum deposits contain petroleum of such a high viscosity that production of this petroleum by primary recovery techniques is impracticable, if not impossible. Indeed, two of the world's largest petroleum accumulations, the vast tar sand deposits in western Canada and Venezuela, remain essentially untapped because of the high viscosity of the petroleum contained therein. In order to successfully recover this kind of petroleum, at least three objectives must be achieved. First, fluid flow communication must be established between injection and producing wells. Second, the viscosity of the petroleum must be reduced to a level sufficient to allow flow of the petroleum through the reservoir, and third, the recovery method must be able to act upon a large portion of the pore volume of the reservoir.
Thermal oil recovery methods such as in situ combustion and steam flooding have been shown to be effective in mobilizing the hydrocarbons in high viscosity petroleum reservoirs. Steam flooding has enjoyed increasing popularity in recent years and is the most commercially successful of the thermal oil recovery techniques. Steam flooding can be utilized in a single well by the so called "huff-and-puff" technique. This method involves first injecting steam into a well, then shutting in the well for a "soak" period wherein the heat contained in the steam raises the temperature and lowers the viscosity of the petroleum, and then a production period wherein the mobilized petroleum is produced from the well, usually by pumping. Steam flooding may also be utilized as a steam drive or steam through-put process, wherein steam is injected into the reservoir through one or more injection wells. This steam then moves through the subterranean reservoir mobilizing and volatilizing the petroleum it encounters. This steam-flood front moves through the reservoir towards the production wells, wherefrom the petroleum fluids are produced. The steam drive process is more effective than the "huff-and-puff" method inasmuch as the potential volume of the reservoir which can be swept by process is much greater.
Although the steam drive process is very effective in recovering petroleum from the portions of the reservoir through which the steam actually sweeps, in practice, the success of the steam drive method is often poor because of the process' inability to develop fluid communication and because of low vertical conformance efficiency. Establishment of fluid communication between injection and production wells is especially difficult in these high viscosity petroleum reservoirs. Low vertical fracture pressures in many reservoirs prevent the use of high pressures during the injection process. Also in thermal recovery methods the heated and mobilized petroleum will form banks at the leading edge of the thermal flood. If such a bank progresses through the formation to a point far enough away from the injection well that there is no longer sufficient heat and pressure to propagate its continued flow, the bank will cool and solidify into an impermeable barrier to further fluid flow. The major cause of the poor vertical conformance, which is a measure of the portion of the reservoir swept by the process, is the fact that steam, having a lower density than the other fluids present in the reservoir, will tend to migrate to the upper portions of the reservoir, forming channels across the top of the reservoir to the production well. Once these steam channels have formed in the upper portion of the reservoir, the permeability of these zones is increased due to the continued action of the steam upon the formation and subsequent removal of petroleum. In this manner, more and more steam will flow through these high permeability channels directly to the production wells, leaving the great majority of the reservoir essentially untouched by the steam drive process. It is typical that less than 50 percent of the petroleum contained within the formation can be recovered by the steam drive process leaving large amounts of petroleum within the reservoir after the completion of the process. The severity of this vertical conformance problem increases with the thickness of the oil reservoir and with the viscosity of the petroleum contained within the reservoir.
As it can be seen in the above discussion, there remains a substantial need for a steam drive process that can develop fluid communication under adverse conditions and that exhibits improved vertical conformance efficiency resulting in substantial increases in petroleum recovery.